The present disclosure relates to an exposure photo mask used in a projection exposure apparatus and a method for forming a pattern using the same.
In order to improve the performance of semiconductor elements and to reduce a chip area, the degree of integration of semiconductor integrated circuits has been increased. Accordingly, the line width of patterns has been reduced. Therefore, in a lithography process step in which a circuit pattern is formed on a semiconductor substrate, pattern formation with high resolution is required.
Lithography, a resolution pitch RP, and the depth of focus DOF are given by Expression 1 and Expression 2, which are equations called Rayleigh's equations.RP=k1·λ/NA  Expression 1DOF=k2·λ/(NA)2  Expression 2
In Expressions 1 and 2, k1 and k2 are called process factor, and are parameters influenced by an exposure wavelength, a resist type, etc. In Expressions 1 and 2, λ represents the wavelength of exposure light and NA represents the numerical aperture of an exposure apparatus.
On the basis of Expression 1, in order to attain high resolution, the wavelength λ has to be reduced or the numerical aperture NA has to be increased. However, reducing the wavelength requires great change in the process itself, such as the development of a laser device serving as a light source, the development of a resist material that senses light in a wavelength band of the laser device, etc. Thus, a method for achieving high NA is employed in general in order to attain high resolution. However, as understood from Expression 2, increasing NA reduces the depth of focus.
As described above, as a method other than the method in which the wavelength is reduced and the method in which the numerical aperture NA is increased, a technique which enables increase in the resolution, i.e., increase in the contrast of an optical image, and increase in the depth of focus without changing the wavelength λ and the numerical aperture NA at the same time has been important.
The most typical technique, among techniques of increasing the contrast and the depth of focus, is a method in which oblique-incidence exposure is performed on a periodic pattern formed on a photo mask. However, in oblique-incidence exposure, substantially effective advantages are achieved only when patterns are provided at short intervals of λ/NA or less. Therefore, this method is not effective for reducing the size of arbitrary patterns. As a method for making up for the shortage of oblique-incidence exposure, a method (which will be hereinafter referred to as an “auxiliary pattern method”) using an auxiliary pattern may be employed. Note that it is well known that, in the auxiliary pattern method, the focus location of a main pattern and the location where a transcription image is formed are not changed.
The auxiliary pattern method described in Japanese Unexamined Patent Publication No. H05-165194 (which will be hereinafter referred as a first related art example) will be described below. FIG. 16 illustrates a planar configuration of a photo mask used in the first related art example. The photo mask illustrated in FIG. 16 is used, for example, in a exposure apparatus which is capable of performing reduced size projection exposure to reduce the size to one fifth of the original size. As illustrated in FIG. 16, a light shielding film 102 made of chromium (Cr) is attached to a surface of a transparent glass substrate 101 serving as a mask substrate. In the light shielding film 102, an opening portion 103 for a circuit pattern which is a main pattern is formed. Opening portions 104 and 105 for auxiliary patterns are formed at both sides of the opening portion 103 so as to be adjacent to the opening portion 103 for the circuit pattern. In this case, for example, the width of the opening portion 103 is set to 1.5 μm. Each of the respective widths of the opening portions 104 and 105 is set to 0.75 μm. Each of the center distance of each of the opening portion 103 for a circuit pattern and the opening portions 104 and 105 for auxiliary patterns is set to 4.5 μm.
That is, in the photo mask used in the first related art example, auxiliary patterns having a smaller dimension than the dimension of a circuit pattern which is a main pattern are provided at both sides of the main pattern so as to be adjacent to the circuit pattern. However, in the first related art example, although the depth of focus is slightly increased, similar advantages to those of the typical periodic pattern are not achieved.
An improved method of the first related art example, i.e., an auxiliary pattern method described in Japanese Unexamined Patent Publication No. H09-073166 (which will be hereinafter referred to as a second related art example) will be described.
FIG. 17 illustrate a planar configuration of a photo mask used in the second related art example. As illustrated in FIG. 17, a main pattern 202 which is a light shielding portion is provided on a glass substrate 201. Furthermore, a plurality of auxiliary patterns 203 are periodically provided at both sides of the main pattern 202 on the glass substrate 201 at predetermined intervals. The main pattern 202 is formed by a layered film including a low transmittance film as a lower layer and a light shielding film (a chromium film) as an upper layer. Each of the auxiliary patterns 203 is formed by the low transmittance film which is left after the light shielding film as the upper layer has been removed. In this case, the auxiliary pattern 203 formed by the low transmittance film does not form a non-photosensitive portion in a resist film at the time of exposure. Therefore, the depth of focus is increased by periodically providing the auxiliary patterns 203 having a low transmittance, relative to the main pattern 202, and performing oblique-incidence exposure.